Naturally activated adaptive immunity in COVID‐19 patients

Coronavirus disease‐2019 (COVID‐19) caused by severe acute respiratory syndrome coronavirus (SARS‐CoV‐2) has rapidly spread worldwide, threatening the health and lives of many people. Unfortunately, information regarding the immunological characteristics of COVID‐19 patients remains limited. Herein, we collected blood samples from 18 healthy donors (HDs) and 38 COVID‐19 patients to analyse changes in the adaptive immune cell populations and their phenotypes. We observed that the lymphocyte percentage moderately decreased, CD4 and CD8 T cell percentage among lymphocytes were similar, and B cell percentage was increased in COVID‐19 patients in comparison to that in HDs. T cells, especially CD8 T cells, showed an enhanced expression of late activation marker CD25 and exhaustion marker PD‐1. Importantly, SARS‐CoV‐2 infection increased the percentage of T follicular helper– and germinal centre B–like cells in the blood. The parameters in COVID‐19 patients remained unchanged across various age groups. Therefore, we demonstrated that the T and B cells are activated naturally and are functional during SARS‐CoV‐2 infection. These data provide evidence that the adaptive immunity in most patients could be primed to induce a significant immune response against SARS‐CoV‐2 infection upon receiving standard medical care.